In a bowl or measuring cup, combine
the soy sauce, rice wine vinegar, Sriracha sauce, green scallions,
garlic and ginger. Add the marinade over the tuna steaks in a shallow
dish. Cover and let sit in the
fridge for up to 2 hours, spooning the marinade over the tuna
periodically.

Remove the
tuna from the fridge and let sit in the marinade outside of the fridge
for 20 to 30 minutes so the steaks can come up to room temperature. When the tuna
has come to room temperature, remove it from the
marinade. Gently scrape off some of the marinade when ready to sear the
tuna. Sprinkle the top surface with the mixture of sesame seeds.

Bring
your grill pan (or grill) to very high heat. When the pan is VERY hot
but not quite smoking, add the tuna steaks, sesame seed side DOWN, and
sear for about 4 - 5 minutes on the first side.
Don't try to move them during this time. While searing, sprinkle the
top (naked) side of the tuna with additional sesame seeds. Flip the
steaks and sear on the 2nd side for about 3 minutes. The 2nd side will
not take as long to cook as the meat will have begun to heat up by this
point.

Remove the tuna from the pan, and let them rest for 4 to 5 minutes. Garnish with additional green onions if you'd like.

If you desire a
sauce for the tuna, you could bring the marinade to a boil for 2 - 3
minutes in a small saucepan and serve on the side. For a thinner
dipping sauce, boil "as is." For a thicker sauce, add a tiny bit of
cornstarch (1/4 tsp ?) before heating, then bring to a boil. By boiling
for a few minutes, you will kill any bacteria from the raw meat and
render the sauce safe to eat. If this makes you too nervous, you could
just double the marinade recipe and separate it prior to adding the
meat.